OVERVOLATGES AND INSULATION COORDINATION PART -2

Partial Discharge Detection

• A common mode of failure of high voltage cables involves voids or inclusions. 
• These cause partial discharge (pd) activity which, if allowed to continue unchecked, may compromise the integrity of the insulation. 
• Since traditional methods of monitoring pd activity are only suitable for use within a well-screened laboratory, our work has considered a range of alternatives, with the overall aim of developing a system that can detect and locate pd activity on-line. 
• Methods include the use of coupling techniques, radio frequency current transducers, and acoustic emissions. 
• Recent research has concentrated on requirements for systems capable of continuously monitoring high voltage cable circuits, and we have developed a pd data transmission system using electro-optic technology. 
• In addition we are developing techniques for characterizing pd measurement data using genetic algorithms and pattern recognition techniques.

The parameters and the characteristics of the lightning strokes

• Amplitude of the current, the rate of rise, the probability distribution of them and the wave shapes of the lightning voltages and currents.

Isokeraunic level or thunderstorm days

        It is defined as the number of days in a year when the thunder is heard or recorded in a particular location. Often it does not distinguish between the ground strokes and the cloud-to-cloud strokes.

Factors influence the lightning induced voltages on transmission lines

• The ground conductivity

• The leader stroke current 

• The corona

Attenuation and distortion of traveling waves

• The decrease in the magnitude of the wave as it propagates along the line is called attenuation.
• The elongation or change of wave shapes that occur is called distortion.

Over voltages generated in EHV system

        Over voltages are generated in EHV systems when there is a sudden release of internal energy stored either in the electrostatic form or in the electromagnetic form.

Causes for power frequency and its harmonic over voltages

• Sudden loss of loads
• Disconnection of inductive loads or connection of capacitive loads,
• Ferranti effect,
• unsymmetrical faults
• saturation in transformers

Uses of shunt reactors

• Used to limit the voltage rise due to Ferranti effect.
• Used to reduce surges caused due to sudden energizing.

Ground wire

• Ground wire is a conductor run parallel to the main conductor of the transmission line supported on the same tower and earthed at every equally and regularly spaced towers.
• It is run above the main conductor of the line.

Use of ground wire

• It shields the transmission line conductor from induced charges, from clouds as well as from a lightning discharge.

Expulsion gap

              Expulsion gap is a device which consists of a spark gap together with an arc quenching device which extinguishes the current arc when the gap breaks over due to overvoltage.

Parts of an expulsion gap.

                  It consists of a rod gap in air in air in series with a second gap enclosed within a fibre tube.

Protector tube

• It is a device which consists of a rod or spark gap in air formed by the line conductor and its high voltage terminal. It is mounted underneath the line conductor on a tower.
• Selecting the risk of failure, the statistical safety factor and by firing the withstand level of any equipment or apparatus corresponding to 90% or 95% of the withstand voltage.

Basic Impulse Level

• It is defined as the minimum insulation impulse withstands voltage of any power equipment or apparatus.
• The BIL of a power system is usually chosen as 25% to 30% more than the protective level offered by the protective devices.

Main disadvantages of Zinc Oxide arrester

              It is the continuous flow of power frequency current and the consequent power loss.

Various insulation levels in a substation

• The busbar insulation is the highest to ensure the continuity of supply in a substation.
• The circuit breakers, isolators, instrument and relay transformers are given the next lower limiting level.
• The power transformers are the costliest and sensitive device and the insulation level for it is the lowest.

Surge arresters

            They are non-linear resistors in series with spark gaps which act as fast switches.

Various types of surge arresters used for EHV and UHV systems

• Silicon carbide arresters with spark gaps
• Silicon carbide arresters with current limiting gaps
• The gapless metal oxide arresters


Equation of surge admittance and surge impedance of the transmission line

        Y(S)=  C/L ((S+α - β )(S+α + β ))1/2
         Z(S)=  L/C (( S+α-β)(S+α-β))1/2

Where α is the attenuation constant and β is the wavelength constant.
           
Gas- insulating medium in electrical apparatus

        Most of the electrical apparatus use air as the insulating medium and in a few cases other gases such as nitrogen, carbon dioxide, Freon and sulphurhexafluoride

Breakdown voltage

The maximum voltage applied to the insulation at the moment of breakdown is called the breakdown voltage.

Spark breakdown

Spark breakdown is the transition of a non-sustaining discharge into a self-sustaining discharge.
Theories mechanism for breakdown
• Townsend theory
• streamer theory


Mean free path

• It is defined as the average distance between collisions.
• The mean free path is expressed as k/p cm, where k is a constant and p is the gas pressure in microns.

Ionization

          The process of liberating an electron from a gas molecule with a simultaneous production of a positive ion is called ionization.

Types of over voltages in High Voltage Engineering

• Lightning over voltages.

• Switching over voltages.

Oil Paper Insulation Systems

• Although polymers are now the most common insulation materials at low voltages, at high voltages, most insulation is still based largely upon paper and oil. Such insulation systems are used in cables, transformers and bushings. 

• Consequently, understanding their behavior when exposed to high fields and elevated temperatures for prolonged periods are of great practical importance.

• In transformers, for example, the oil serves the dual role of electrical insulation and a medium by which heat dissipated in the transformer windings and core is transferred to the environment.

• Work is currently being undertaken on examining the macroscopic properties of mineral oil subject, to both electrical and thermal ageing. 

• Also, we are exploring the basic chemistry of ageing in oil systems, to enable us to develop simple analytical techniques that can be used in practice to provide an accurate representation of the integrity of high voltage plant. 

• The work is extremely multidisciplinary in nature, involving a combination of electrical measurements and chemical characterization using optical spectroscopes, mass spectrometry, chromatography and related analytical techniques.

VARIOUS REGIONS OF THE CLOUD

• The upper regions of the cloud are positively charged, whereas the lower region and the base are predominantly negative except the local region near the base and the head which is possible.

• A thunder cloud consists of super cooled water droplets moving upwards and large hailstones moving downwards.

• The requirements for the thunder clouds and charge formation of air currents are moisture and specific temperature range

Different theories of charge formation

• Simpson’s theory

• Reynold’s theory

• Mason’s theory

Back flashover

• When a direct lightning stroke occurs on a tower, the tower has to carry huge impulse currents. 

• If the tower footing resistance is considerable, the potential of the tower rises to a large value, steeply with respect to the line and consequently a flashover may take place along the insulator strings .This is known as back flashover.

LIGHTNING ARRESTERS IN HIGH VOLTAGE ENGINEERING

• The earthling screen and ground wires can well protect the electrical system against direct lightning strokes but they fail to provide protection against traveling waves which may reach the terminal apparatus. 

• The lightning arresters or surge diverters provide protection against such surges.

• A lightning arrester or a surge diverter is a protective device which conducts the high voltage surges on the power system to the ground.

  

• Figure shows the basic form of a surge diverter. It consists of a spark gap in series with a non-linear resistor. One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded. 

• The length of the gap is so set that normal line voltage is not enough to cause an arc across the gap but a dangerously high voltage will break down the air insulation and form an arc. 

• The property of the non-linear resistance is that its resistance decreases as the voltage (or current) increases and vice-versa. This is clear from the volt/amp characteristic of the resistor shown in Figure. 

Action: The action of the lightning arrester or surge diverter is as under:

(i) Under normal operation, the lightning arrester is off the line i.e., it conducts r’ no current to earth or the gap is non-conducting.

(ii) On the occurrence of over voltage, the air insulation across the gap breaks down and an arc is formed, providing a low resistance path for the surge to the ground.

(iii) In this way, the excess charge on the line due to the surge is harmlessly conducted through the arrester to the ground instead of being sent back over the line.

(iv) It is worthwhile to mention the function of non-linear resistor in the operation of arrester. 

(v) As the gap sparks over due to over voltage, the arc would be a short circuit on the power system and may cause power-follow current in the arrester. 

Since the characteristic of the resistor is to offer high resistance to high voltage (or current), it prevents the effect of a short circuit. After the surge is over, the resistor offers high resistance to make the gap non-conducting.

• Two things must be taken care of in the design of a lightning arrester. 

• Firstly, when the surge is over, the arc in gap should cease. If the arc does not go out, the current would continue to flow through the resistor and both resistor and gap may be destroyed. 

• Secondly, IR drop (where I is the surge current) across the arrester when carrying surge current should not exceed the breakdown strength of the insulation of the equipment to be protected.

TYPES OF LIGHTNING STROKES IN HIGH VOLTAGE ENGINEERING

There are two main ways in which a lightning may strike the power system (e.g. overhead lines, towers, sub-stations etc.), namely;
1. Direct stroke
2. Indirect stroke

1. Direct stroke: In the direct stroke, the lightning discharge (i.e. current path) is directly from the cloud to the subject equipment e.g. an overhead line. From the line, the current path may be over the insulators down the pole to the ground.

The over voltages set up due to the stroke may be large enough to flashover this path directly to the ground. The direct strokes can be of two types viz. (i) Stroke A and (ii) stroke B.




(i) In stroke A, the lightning discharge is from the cloud to the subject equipment i.e. an over head line in this case as shown in Figure.

• The cloud will induce a charge of opposite sign on the tall object (e.g. an overhead line in this case). When the potential between the cloud and line exceeds the breakdown value of air, the lightning discharge occurs between the cloud and the line.

(ii) In stroke B, the lightning discharge occurs on the overhead line as a result of stroke A between the clouds as shown in Figure. There are three clouds P, Q and R having positive, negative and positive charges respectively.

• The charge on the cloud Q is bound by the cloud R. If the cloud P shifts too near the cloud Q, then lightning discharge will occur between them and charges on both these clouds disappear quickly. 

• The result is that charge on cloud R suddenly becomes free and it then discharges rapidly to earth, ignoring tall objects.

2. Indirect stroke:  Indirect strokes result from the electro statically induced charges on the conductors due to the presence of charged clouds. This is illustrated in Figure.

• A positively charged cloud is above the line and induces a negative charge on the line by electrostatic induction. This negative charge, however, will be only on that portion of the line right under the cloud and the portions of the line away from it will be positively charged as shown in Figure.

• The induced positive charge leaks slowly to earth via the insulators. When the cloud discharges to earth or to another cloud, the negative charge on the wire is isolated as it cannot flow quickly to earth over the insulators.

• The result is that negative charge rushes along the line is both directions in the form of travelling waves. It may be worthwhile to mention here that majority of the surges in a transmission line are caused by indirect lightning strokes.

HARMFUL EFFECTS OF LIGHTNING:

• A direct or indirect lightning stroke on a transmission line produces a steep-fronted voltage wave on the line. 

• The voltage of this wave may rise from zero to peak value (perhaps 2000 kV) in about I is and decay to half the peak value in about Sits. Such a steep-fronted voltage wave will initiate travel ling waves along the line in both directions with the velocity dependent upon the Land C parameters of the line.

(i) The traveling waves produced due to lightning surges will shatter the insulators and may even wreck poles.

(ii) If the traveling waves produced due to lightning hit the windings of a transformer or generator, it may cause considerable damage. The inductance of the windings opposes any sudden passage of electric charge through it.

Therefore, the electric charges “up” against the transformer (or generator). This induces such an excessive pressure between the windings that insulation may breakdown, resulting in the prod of arc. While the normal voltage between the turns is never enough to start an arc; once the insulation has broken down and an arc has been started by a momentary over voltage, the line voltage is usually sufficient to maintain the arc long enough to severely damage the machine.

(iii) If the arc is initiated in any part of the power system by the lightning stroke, this arc will set up very disturbing oscillations in the line. This may damage other equipment connected to the line.

LIGHTNING PHENOMENA IN HIGH VOLTAGE ENGINEERING

An electric discharge between cloud and earth, between clouds or between the charge centers of the same cloud is known as lightning.

• Lightning is a huge spark and takes place when clouds are charged to such a high potential (+ve or -ve) with respect to earth or a neighboring cloud that the dielectric strength of neighboring medium (air) is destroyed. 

• There are several theories which exist to explain how the clouds acquire charge. The most accepted one is that during the up rush of warm moist air from earth, the friction between the air and the tiny particles of water causes the building up of charges.

• When drops of water are formed, the larger drops become positively charged and the smaller drops become negatively charged. 

• When the drops of water accumulate, they form clouds, and hence cloud may possess either a positive or a negative charge, depending upon the charge of drops of water they contain. 

• The charge on a cloud may become so great that it may discharge to another cloud or to earth and we call this discharge as lightning. The thunder which accompanies lightning is due to the fact that lightning suddenly heats up the air, thereby causing it to expand. 

• The surrounding air pushes the expanded air back and forth causing the wave motion of air which we recognize as thunder.

MECHANISM OF LIGHTNING DISCHARGE:

• When a charged cloud passes over the earth, it induces equal and opposite charge on the earth below. Figure shows a negatively charged cloud inducing a positive charge on the earth below it. 

• As the charge acquired by the cloud increases, the potential between cloud and earth increases and, therefore, gradient in the air increases. 

• When the potential gradient is sufficient (5 kV/cm to 10 kv/cm) to break down the surrounding air, the lightning stroke starts. 

• The stroke mechanism is as under:(i) As soon as the air near the cloud breaks down, a streamer called leader streamer or pilot streamer starts from the cloud towards the earth and carries charge with it as shown in Figure. 

•  The leader streamer will continue its journey towards earth as long as the cloud, from which it originates, feeds enough charge to it to maintain gradient at the tip of leader streamer above the strength of air. 

•  If this gradient is not maintained, the leader streamer stops and the charge is dissipated without the formation of a complete stroke. 

(ii) In many cases, the leader streamer continues its journey towards earth. As the leader streamer moves towards earth, it is accompanied by points of luminescence which travel in jumps giving rise to stepped leaders.

• The velocity of stepped leader exceeds one-sixth of that of light and distance traveled in one step is about 50 m. It may be noted that stepped leaders have sufficient luminosity and give rise to first visual phenomenon of discharge.

(iii) The path of leader streamer is a path of ionization and, therefore, of complete breakdown of insulation. 

• As the leader streamer reaches near the earth, a return streamer shoots up from the earth to the cloud, following the same path as the main channel of the downward leader. 

• The action can be compared with the closing of a switch between the positive and negative terminals; the downward leader having negative charge and return streamer the positive charge. 

This phenomenon causes a sudden spark which we call lightning. With the resulting neutralization of much of the negative charge on the cloud, any further discharge from the cloud may have to originate from some other portion of it.

The following points may be noted about lightning discharge:

(a) A lightning discharge which usually appears to the eye as a single flash is in reality made up of a number of separate strokes that travel down the same path. The interval between them varies from 09005 to 05 second. Each separate stroke starts as a downward leader from the cloud.

(b) It has been found that 87% of all lightning strokes result from negatively charged clouds and only 13% originate from positively charged clouds.

(c) It has been estimated that throughout the world, there occur about 100 lightning strokes per second.

(d) Lightning discharge may have currents in the range of 10 kA to 90 kA.

Natural causes of over voltages in HV

• There are several instances when the elements of a power system (e.g. generators, transformers, transmission lines, insulators, etc.) are subjected to over voltages i.e. voltages greater than the normal value.

• These over voltages on the power system may be caused due to many reasons such as lightning, the opening of a circuit breaker, the grounding of a conductor etc.

• Most of the over voltages are not of large magnitude but may still be important because of their effect on the performance of circuit interrupting equipment and protective devices.

• An appreciable number of these over voltages are of sufficient magnitude to cause insulation breakdown of the equipment in the power system.

• Therefore, power system engineers always device ways and means to limit the magnitude of the over voltages produced and to control their effects on the operating equipment. 

VOLTAGE SURGE:

• A sudden rise in voltage for a very short duration on the power system is known as a voltage surge or transient voltage. 

• Transients or surges are of temporary nature and exist for a very short duration but they cause over voltages on the power system.

• They originate from switching and from other causes but by far the most important transients are those caused by lightning striking a transmission line. 

• When lightning strikes a line, the surge rushes along the line, just as a flood of water rushes along a narrow valley when the retaining wall of a reservoir at its head suddenly gives way.

CAUSES OF OVERVOLTAGES:

The over voltages on a power system may be broadly divided into two main categories viz.

1. Internal causes:

(i) Switching surges

(ii) Insulation failure

(iii) Arcing ground

(iv) Resonance

2. External causes (ie.) Lightning:

Internal causes:

• Internal causes do not produce surges of large magnitude. Experience shows that surges due to internal causes hardly increase the system voltage to twice the normal value. 

• Generally, surges due to internal causes are taken care of by providing proper insulation to the equipment in the power system. 

• However, surges due to lightning are very severe and may increase the system voltage to several times the normal value. If the equipment in the power system is not protected against lightning surges, these surges may cause considerable damage. 

• In fact, in a power system, the protective devices provided against over voltages mainly take care of lightning surges.

INTERNAL CAUSES OF OVERVOLTAGES:

• Internal causes of over voltages on the power system are primarily due to oscillations set up by the sudden changes in the circuit conditions. 

• This circuit change may be a normal switching operation such as opening of a circuit breaker, or it may be the fault condition such as grounding of a line conductor. 

• In practice, the normal system insulation is suitably designed to withstand such surges. 

1. Switching Surges: The over voltages produced on the power system due to switching operations are known as switching surges. 

(i) Case of an open line: During switching operations of an unloaded line, traveling waves are set up which produce over voltages on the line.

• When the unloaded line is connected to the voltage source, a voltage wave is set up which travels along the line.

• On reaching the terminal point A, it is reflected back to the supply end without change of sign.

• This causes voltage doubling i.e. voltage on the line becomes twice the normal value. 

• If Er.m.s is the supply voltage, then instantaneous voltage which the line will have to withstand will be 22E. This over voltage is of temporary nature.

• It is because the line losses attenuate the wave and in a very short time, the line settles down to its normal supply voltage E. 

• Similarly, if an unloaded line is switched off, the line will attain a voltage of 22E for a moment before settling down to the normal value.

(ii) Case of a loaded line: Over voltages will also be produced during the switching operations of a loaded line. Suppose a loaded line is suddenly interrupted. 

(iii) Current chopping: Current chopping results in the production of high voltage transients across the contacts of the air blast circuit breaker.

• Unlike oil circuit breakers, which are independent for the effectiveness on the magnitude of the current being interrupted, air-blast circuit breakers retain the same extinguishing power irrespective of the magnitude of this current.

• When breaking low currents (e.g. transformer magnetizing current) with air-blast breaker, the powerful de-ionizing effect of air-blast causes the current to fall abruptly to zero well before the natural current zero is reached.

• This phenomenon is called current chopping and produces high transient voltage across the breaker contacts. Over voltages due to current chopping represented by resistance switching.

2. Insulation failure: The most common case of insulation failure in a power system is the grounding of conductor (i.e. insulation failure between line and earth) which may cause over voltages in the system. 

Suppose a line at potential E is earthed at point X. The earthing of the line causes two equal voltages of —E to travel along XQ and XP containing currents —E/Zn and +E/Zn respectively. Both these currents pass through X to earth so that current to earth is 2 E/Zn.

3. Arcing ground: In the early days of transmission, the neutral of three phase lines was not earthed to gain two advantages. Firstly, in case of line-to-ground fault, the line is not put out of action. 

• Secondly, the zero sequence currents are eliminated, resulting in the decrease of interference with communication lines. Insulated neutrals give no problem with short lines and comparatively low voltages. 

• However, when the lines are long and operate at high voltages, serious problem called arcing ground is often witnessed. The arcing ground produces severe oscillations of three to four times the normal voltage.

• The phenomenon of intermittent arc taking place in line-to-ground fault of a 34 system with consequent production of transients is known as arcing ground.

• The transients produced due to arcing ground are cumulative and may cause serious damage to the equipment in the power system by causing breakdown of insulation, Arcing ground can be pre vented by eat-thing the neutral.

4. Resonance:  Resonance in an electrical system occurs when inductive reactance of the circuit becomes equal to capacitive reactance. 

• Under resonance, the impedance of the circuit is equal to resistance of the circuit and the p.f. is unity. Resonance causes high voltages in the electrical system. 

• In the usual transmission lines, the capacitance is very small so that resonance rarely occurs at the fundamental supply frequency. 

• However, if generator e.m.f. wave is distorted, the trouble of resonance may occur due to 5th or higher harmonics and in case of underground cables too.

OVERVOLATGES AND INSULATION COORDINATION IN HV

INTRODUCTION:

• In early days, there was a little demand for electrical energy so that small power stations were built to supply lighting and heating loads. 

• However, the widespread use of electrical energy by modern civilization has necessitated producing bulk electrical energy economically and efficiently. 

• The increased demand of electrical energy can be met by building big power stations at favorable places where fuel (coal or gas) or water energy is available in abundance. 

• This has shifted the site of power stations has to be supplied to the consumers. 

• The electrical energy produced at the power stations has to be supplied to the consumers. There is a large network of conductors between the power station and the consumers. 

Natural causes of over voltages

Lightning phenomena

Over voltages due to switching surges

System faults and other abnormal conditions

Principles of insulation co-ordination